JPH11350036A - Method for giving residual compressive stress to bearing ring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve service life of a bearing assembling a bearing ring by giving the residual compressive stress having not less than a prescribed value to the bearing ring without applying an after-treatment of machining etc., and without increasing the producing time of bearing ring. SOLUTION: The bearing ring 1 executing quenching, is applied with a tempering in the state of restricted with a die 2 having smaller thermal expansion coefficient than that of the bearing ring 1 and shrunk and the residual compressive stress is given to the bearing ring 1 at the same time of tempering. At this time, the heat treatment condition of the tempering is adjusted so that the shrinkage ratio of the bearing ring 1 after treating becomes >=0.1%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing bearing steel, and more particularly to a method for applying a residual compressive stress to a bearing race in order to improve the life of the bearing.

[0002]

2. Description of the Related Art Conventionally, bearing rings have been subjected to quenching and tempering in order to provide desired hardness and toughness (toughness).

However, conventionally, a tensile stress of about 50 [MPa] remains on the surface of the orbital ring after tempering, and this residual tensile stress adversely affects the bearing life. For the purpose of improving the life, a post-treatment for giving a residual compressive stress is performed.

[0004] A conventional general method of applying a residual compressive stress is, for example, as described in Japanese Patent Application Laid-Open No. 5-163526, in which a raceway of a bearing after quenching and tempering is ground, By applying machining such as turning and shot peening, residual compressive stress is imparted.

Conventionally, the alternator housing is made of aluminum for the purpose of weight reduction, but downsizing of the bearing is required for the purpose of further weight reduction and size reduction.

However, if the outer diameter of the outer ring of the bearing is reduced for downsizing of the bearing, the thickness of the outer ring becomes smaller, and if the outer ring is used in a housing made of aluminum having low rigidity, the alternator is not used. Bending stress is applied to the outer ring of the front bearing (NSK Technic)
a1 Journal N0.656 (1993) p.
10). The fatigue life under this bending stress is shorter than that when no bending stress is applied.

In order to extend the life of rolling bearings used under bending stress, such as the alternator bearing, carburized steel is subjected to carburizing treatment, or fully hardened steel is subjected to the above-described machining. Then, it is conceivable to apply residual compressive stress to the raceway surface, and it is considered effective to apply the residual compressive stress to the raceway surface in this manner (Preliminary Proceedings of the Tribology Conference of the Japan Society of Tribology, Nagoya, 1993) 11).

Further, in a self-aligning roller bearing or the like used in a paper machine or the like, in order to prevent creep occurring between the shaft and the bearing inner ring, a one-to-one connection between the shaft and the inner diameter of the bearing inner ring.
It may be used by giving a high fitting stress exceeding 00 [MPa]. In such a case, in order to easily apply a fitting stress, an inner ring whose inner diameter is processed into a tapered shape is usually pressed into a tapered shaft. As the inner ring having such a shape, generally used is one obtained by quenching and tempering completely hardened steel such as high carbon chromium bearing steel.

The inner ring made of fully hardened steel has 100
When used with a fitting stress exceeding [MPa],
Due to the combination of the fitting stress and the rolling stress, the inner ring may be broken in the axial direction starting from a non-metallic inclusion or the like existing near the raceway surface of the inner ring.

In order to prevent such breakage of the inner race, it is effective to apply residual compressive stress to the raceway surface and to increase the fracture toughness of the material itself. The residual compressive stress on the raceway surface is increased by performing austempering or using carburized steel.

[0011]

However, the bearing ring subjected to the heat treatment has a very hard surface, so that the processing conditions become severe in grinding and turning, and there is a problem in terms of grinding burn and tool life. Occurs. In addition, surface processing such as shot peening takes a long time, and is inefficient as processing for applying a compressive stress to the surface.

In the carburizing hardening method using carburized steel, a residual compressive stress of about 200 [MPa] can be applied to the raceway surface by controlling the conditions of carburizing, quenching and tempering. And the extension effect of 130
Although it is effective for preventing breakage of an inner ring used under a fitting stress exceeding [MPa], the time required for carburizing becomes long, and particularly, the carbon content of about 0.2 [vol%] is small. When the carburizing treatment is performed on the steel material, there is a problem that the time required for the carburizing treatment becomes longer. There was a problem.

In the method of performing austempering on the fully hardened steel, the residual compressive stress that can be applied to the inner raceway surface by the austempering is around 100 [MPa]. What
When used under a higher fitting stress, the inner ring cannot be prevented from breaking.

The present invention has been made in view of the above-mentioned problems, and without performing post-processing such as machining.
In other words, a method for applying residual compressive stress to a bearing race that can provide a residual compressive stress equal to or more than a predetermined value to the race without increasing the manufacturing time of the bearing race and can improve the life of the bearing incorporating the race. It is an object to provide

[0015]

Means for Solving the Problems In order to solve the above-mentioned problems, a method for applying residual compressive stress to a bearing race according to the present invention comprises:
A method of shrinking a bearing ring of a quenched bearing by performing tempering in a state of being constrained by a mold having a smaller coefficient of thermal expansion than that of the bearing ring to impart residual compressive stress to the bearing ring, By adjusting the heat treatment conditions for tempering, the shrinkage of the bearing ring is reduced by 0.1% by a shrinkage ratio S defined by the following equation.
The features are as described above.

[0016] Here, “after shrinkage” refers to a state when cooled to a room temperature state.

According to the present invention, at the time of conventional tempering, the mold is restrained by a mold having a smaller thermal expansion coefficient than that of the bearing ring at the time of the tempering, so that it is retained inside the bearing ring at the same time as the tempering. Compressive stress can be applied.

Although the residual compressive stress level that can be applied is lower than that of the carburizing method, the deformation of the bearing ring can be suppressed less than that of the carburizing method because the mold is constrained.
Moreover, equipment costs are reduced.

Further, by setting the shrinkage ratio S to 0.1% or more, a residual compressive stress of 100 [MPa] or more can be applied to at least the inner ring of a generally used bearing as described later. As a result, the life of the bearing is reliably improved (see Tables 1 and 2 described later).

Further, even for the inner ring, 100 [MP]
a] When the above-mentioned fitting stress is applied at the time of use, it is preferable to set the shrinkage ratio S to 0.25% or more. By setting the shrinkage ratio S to 0.25% or more,
As will be described later, the residual compressive stress becomes 160 [MPa] or more, and the inner ring crack can be prevented (see FIGS. 8 and 9).

When the outer ring is used in a state where bending stress is applied, the shrinkage ratio S is set to 0.18.
% Is preferably set. Shrinkage S 0.18
%, The residual compressive stress becomes 103 [MPa] or more, as will be described later, and the life of the bearing can be reliably improved even when the bearing is used under bending stress (see FIG. 5 and FIG. 5). (See FIG. 7).

Regarding the adjustment of the shrinkage rate, it is possible to set a shrinkage rate higher than desired by adjusting the heating temperature among the heat treatment conditions for tempering, and the shrinkage rate increases as the heating temperature increases. Can be set.

In the above equation (1), when the inner diameter of the mold is set equal to the outer diameter of the raceway before contraction, the inner diameter of the mold is equal to the outer diameter of the raceway before contraction. . Here, from the viewpoint of imparting the residual compressive stress and improving the bearing life, the higher the shrinkage ratio S, the better, but the upper limit of the tempering temperature from the original purpose of tempering such as the dimensions of the bearing ring as a product and toughness. That is, the upper limit of the contraction rate S is limited.

[0024]

Next, embodiments of the present invention will be described with reference to the drawings. As shown in FIG. 1 which is a process drawing, the manufacturing of the bearing race of the present embodiment includes a quenching step (step 1) of quenching the race, and a quenched race material obtained based on the present invention. A tempering step (step 2) of performing tempering accompanied by application of compressive stress.

In the following description, the tempering according to the present invention is appropriately referred to as compressive stress application to distinguish it from conventional tempering. The quenching step (step 1) is the same as the conventional one. For example, quenching of a bearing ring material made of bearing steel or carburizing / carbonitriding and quenching of a bearing ring material made of case-hardened steel is performed by a heat treatment furnace. Is done.

In the compressive stress applying step (step 2), the outer diameter of the race is constrained by a mold, and the race is heated and cooled in this mold constrained state, so that the race is oriented in the inner diameter direction when compressive stress is applied. Shrink to give residual compressive stress.

An example of the configuration of the apparatus used in the compressive stress applying step according to the present embodiment will be described with reference to FIG. 2 which is a schematic diagram. And a high-frequency (induction heating) coil 3 for orbital heating for heating the orbital ring 1 which is disposed outside the die 2 and which heats the orbital ring 1.

The mold 2 is made of a material (for example, ceramics) that is hardly subjected to induction heating and has a smaller coefficient of thermal expansion than the material of the bearing ring 1. Then, after the bearing ring 1 is fitted into the mold 2, the heat treatment condition for tempering is set to a predetermined value, and the bearing ring 1 restrained by the mold 2 is heated. 2 and contracts in the subsequent cooling step, and a predetermined residual compressive stress is applied simultaneously with tempering.

As described above, in the present embodiment, even if mechanical processing such as grinding as post-processing is not performed, a predetermined residual compressive stress is applied simultaneously with the application of compressive stress, thereby improving the fatigue life of the bearing. Can be.

Here, the means for heating the bearing ring 1 does not need to be limited to induction heating, and other heating means may be used. Then, as shown in the examples described later, by adjusting the heating temperature of the tempering so that the shrinkage ratio becomes 0.1% or more, at least the life of the generally used inner ring is improved, and the temperature is reliably increased. The life of the bearing can be improved.

Further, as described above, despite the application of the residual compressive stress, a post-process (such as machining) is not necessarily required, and the manufacturing time of the race 1 is shortened. The deformation of the bearing ring 1 due to the application of the residual compressive stress can be suppressed to be small as compared with the case of FIG.

Here, the relationship between the shrinkage rate and the heat treatment conditions at the time of tempering can be determined by conducting a test for determining the relationship between the heating temperature and the shrinkage rate in advance for the target raceway material, By calculating the relationship, the shrinkage ratio can be accurately controlled.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following embodiments, the inner diameter of the mold is set to a value substantially equal to the average outer diameter of the race. The step of applying compressive stress was performed by inserting a bearing ring into a mold and then performing induction heating. [First Embodiment] A first embodiment will be described with reference to the drawings.

In the first embodiment, a plurality of inner rings (track rings) of the ball bearing # 6206 formed by turning were prepared and subjected to the quenching and compressive stress applying (tempering) steps described in the above embodiment. At this time, various heat treatment conditions for tempering are set so that the shrinkage rate of each inner ring becomes a target value.

Here, the turning dimensions of each race were adjusted in accordance with the set shrinkage ratio so that the dimensions of the race after the compressive stress applying step were substantially the same. The material is SUJ2
The heat treatment condition is quenching at 850 ° C. for 30 minutes → 60 ° C./cooling in oil.

The surface hardness of the bearing ring after the compressive stress applying step was all HRC 60 to 63. And
The residual stress and the shrinkage rate of each inner ring after the application of the compressive stress were measured, and the relationship between the residual stress and the shrinkage rate was obtained. The result shown in FIG. 3 was obtained.

In FIG. 3, the contraction rate [%] on the horizontal axis is
(Outer diameter before shrinkage−Outer diameter after shrinkage) ÷ (Inner diameter of mold whose outer diameter is restricted) × 100. However, in this embodiment, the inner diameter of the mold is divided by the outer diameter before shrinkage.

The residual stress is measured in the raceway groove outer diameter side after the compressive stress applying step. In the residual stress on the vertical axis, the compression direction is represented by a negative value. The same applies to other embodiments described below.

As can be seen from FIG. 3, the shrinkage ratio and the residual stress are in a linear proportional relationship.
At [%], the residual stress becomes -100 [MPa]. Therefore, by adjusting the shrinkage, the applied residual stress can be adjusted to a desired value.

Next, a plurality of inner rings (track rings) having different shrinkage ratios are manufactured as described above, and the inner rings are connected to ball bearings # 6.
After assembling as 206 and conducting a life test,
The results shown in Table 1 were obtained.

[0041]

[Table 1]

Here, the life test of the ball bearing is performed by using the following bearing load:
5.9 [kN], maximum contact pressure: 3.6 [GPa], number of test bearings: 15 each. Further, the L10 life ratio in Table 1 is a ratio of the life between the present embodiment and the conventional bearing. N0.1 in Table 1 is a bearing subjected to conventional quenching and tempering treatment. The same applies to Table 2.

As can be seen from Table 1, the shrinkage rate was set at 0.
By setting it to 1% or more, the effect of extending the life of the bearing is surely exhibited, and it is understood that the longer the contraction rate is, the longer the life is.

In other words, when the tempering treatment according to the present invention is employed, the occurrence of fatigue cracks and
It is possible to obtain a bearing race ring that is strong against progress and thus has an improved fatigue life.

In the life test of the above embodiment, the present invention is applied only to the inner ring, but the present invention may be applied to the outer ring. Second Embodiment A second embodiment will be described with reference to the drawings.

In the second embodiment, as in the first embodiment, the relationship between the shrinkage ratio and the residual stress and the bearing life test were performed on the inner ring of # 6206. However, the material is S
The heat treatment conditions were 430 ° C. × 120 minutes of carburizing treatment with 820 ° C. × 40 minutes → 60 ° C. quenching by cooling in oil. The surface hardness of the bearing ring after the compression applying step was HRC60 to 63 in all cases.

FIG. 4 shows the relationship between the shrinkage ratio and the residual stress, and Table 2 below shows the results of the life test. The test conditions and the like are the same as in the first embodiment.

[0048]

[Table 2]

As can be seen from FIG. 4, as in the first embodiment, there is a proportional relationship between the shrinkage and the residual stress. Further, before the compressive stress applying step, the surface is coated with 200% by carburizing.
Since the compressive stress of [MPa] or more has already been applied, the residual compressive stress after the compressive stress applying step is larger than that of the first embodiment.

Also, as can be seen from Table 2, the larger the shrinkage ratio, the longer the life of the bearing and the smaller the shrinkage ratio is 0.1.
%, It can be seen that the life of the bearing is surely improved as compared with the conventional example.

Note that only the compressive stress applying step
The applied amount (increase amount) of the residual compressive stress is such that the shrinkage rate is 0.1%.
In this case, 350-210 = 140 [MPa], which is larger than that of the first embodiment in which the carburizing treatment is not performed. Case) is larger.

Here, in the second embodiment, the compressive stress applying step is performed after quenching, but a conventional tempering treatment is separately performed before and after the compressive stress applying step as long as the surface hardness is satisfied. May be implemented. [Third Embodiment] Next, a third embodiment will be described with reference to the drawings.

This embodiment relates to a case where the present invention is applied to an outer ring (track ring) of a rolling bearing used under bending stress, and considers the improvement of the fatigue life of the outer ring.

In the third embodiment, the same measurements and the like were performed under the same processing conditions as in the first embodiment, but the ball bearing formed by turning, instead of the inner ring, was subjected to compressive stress. This is for the outer ring (track ring) of # 6006.

That is, a plurality of outer rings were prepared and subjected to the quenching and compressive stress applying steps described in the above embodiment. Here, the material of the outer ring material is SUJ2, and the heat treatment condition is 850 ° C. × 30 minutes → 60 ° C., quenching by cooling in oil.

The surface hardness of the bearing ring after the compressive stress applying step was all HRC 60 to 63. And
Residual stress was measured for each outer ring after the compressive stress applying step, and the relationship between the residual stress and the above-mentioned shrinkage ratio was obtained. The result shown in FIG. 5 was obtained.

As can be seen from FIG. 5, the residual compressive stress increases as the shrinkage ratio increases. For example, when the shrinkage / shrinkage ratio is 0.18 [%], the residual stress is about -103 [MP].
a].

Next, in order to test the durability of use in a state where a bending stress is applied, a ball bearing incorporating the above outer ring is prepared, and an alternator actual machine test is performed to determine the relationship between residual stress and L10 life. Was examined. FIG. 6 shows a state where the test bearing is incorporated in an alternator actual machine.

FIG. 7 shows the result. Here, as a comparative example, a test bearing was prepared by assembling a quenched and tempered product according to a conventional method with a commonly used material of SUJ2.
As an example, a test was conducted by incorporating the outer ring having a shrinkage factor of 0.18 [%] into the front bearing in FIG.

The test conditions are a bearing load of 2.4 kN and a rotation speed of 18000 [rpm]. The damage of the bearing is checked with a vibrometer. The test was stopped when the value increased to 5 times the initial value.

As can be seen from FIG. 7, in the bearing of the comparative example, the life of L10 with a bending stress of 20 [MPa] applied is reduced by about 40% as compared with the bearing without the bending stress applied. On the other hand, when the bearing of the embodiment is used, when a bending stress of 20 [MPa] is applied,
Although the service life is slightly shorter than that without the application of bending stress, no remarkable difference is observed.

This is because 103 [MP]
It is considered that the effect is due to the slow propagation of fatigue cracks due to the addition of residual compressive stress.
Further, since the compressive stress applying step also has an effect of tempering, if the temperature during the compressive stress applying step is increased, the effect of tempering appears, and a high degree of dimensional stability can be obtained. That is, the amount of retained austenite is reduced to prevent the dimensional expansion over time, and the compressive stress of martensite is sufficiently applied to prevent the dimensional shrinkage over time.

Here, in an environment where bending stress is applied to an alternator or the like, the outer ring is subjected to the greatest bending stress load. ] The above residual compressive stress is applied, that is,
According to the invention of the present application, by applying a compressive stress to the outer ring at the same time as giving the outer ring a shrinkage of 0.18% or more, the outer ring has an outer ring that can withstand use under bending stress.
A rolling bearing can be provided. [Fourth Embodiment] Next, a fourth embodiment will be described with reference to the drawings.

This embodiment is applied to an inner ring. For a rolling bearing used by applying a fitting stress (an average stress in the cross section of the inner ring due to a tensile force applied in a circumferential direction of the inner ring), a high fitting stress This is a study of the breakage of the inner ring, which was liable to occur in the past when used.

In this embodiment, a residual compressive stress was applied to the inner race by the same processing as in the above embodiments. Here, the material of the inner ring is SUJ2, and the heat treatment condition is 850 ° C. × 30 minutes → hardening by cooling in oil at 60 ° C.

The surface hardness of the bearing ring after the compressive stress applying step was HRC 60 to 62 in all cases. And
When the residual stress was measured for the raceway surface of the inner ring contracted to various contractions, the results shown in FIG. 8 were obtained.

As can be seen from FIG. 8, the residual compressive stress increases as the shrinkage rate increases. For example, the residual stress becomes -160 [MPa] at a shrinkage rate of 0.25 [%].

Next, the inner ring 10 having various shrinkage rates, that is, to which various residual compressive stresses are applied, is assembled into a cylindrical roller bearing (NU216; outer diameter φ140 mm, inner diameter φ80 mm, width 26 mm) (FIG. 10). Investigation of the relationship between residual stress and inner ring splitting life yielded the results shown in FIG.

Here, the inner ring split life [re] on the vertical axis of FIG.
v] was determined by an inner ring split life test shown below. In addition, although bearings in which cracking of the inner ring due to fitting stress poses a problem are mainly self-aligning roller bearings, in this embodiment, the cylindrical roller bearing (NU216) is conveniently used as a model of the cracking life of the inner ring as described above. It was used in a typical way.

In addition, bearings in which cracking is a problem
It is used in the state of high fitting stress (between the shaft and the inner ring of inner diameter) exceeding 00 [MPa @ 10 kg / mm ² ], and the inner diameter is tapered to make it easier to apply the stress. is there. The tapered shape aims at a kind of wedge effect.

Next, the procedure of the inner ring split life test will be described. First, a semielliptical fatigue pre-crack 11 is introduced in the axial direction at the center of the raceway surface of the inner ring 10 (see FIG. 10).

For the introduction of the fatigue pre-crack 11, first, a semi-elliptical notch 12 as shown in FIG. 11 is subjected to electric discharge machining in the axial direction at the center of the raceway surface of the inner ring 10. The notch 12 has, for example, a width of 2.5 mm in the axial direction and a depth of 1.0 ± 1.0 mm.
0.1 mm. Next, the inner ring 10 provided with the notch 12
Is installed in a three-point bending tester as shown in FIG. 12, and a load is repeatedly applied to introduce a fatigue pre-crack 11 having a depth of about 0.2 mm from the bottom surface of the notch 12.

Next, after the inner ring 10 is subjected to quenching and tempering processing according to the present invention, that is, contracted by a predetermined shrinkage rate at the time of compressive stress application processing, the inner ring 1 is contracted.
0 is given a predetermined residual compressive stress.

Next, the inner ring 10 with the pre-fatigue crack 11 introduced
The notch 12 is removed by leaving the pre-fatigue crack 11 by grinding the outer diameter surface of. In FIG. 12, the position indicated by the broken line is an example of the grinding depth.

Then, the inner diameter surface of the inner ring 10 is ground to obtain 1 /
After a taper of 12 (組 み 込 む 4.8 °) is formed, the taper is assembled as the inner ring 10 of the cylindrical roller bearing (NU216) (FIG. 10).
reference).

Next, the cylindrical roller bearing was mounted on a rotation tester, and the fitting stress on the inner ring 10 was set to 200 [MP].
a], and the hoop stress is applied, the bearing is rotated at a radial load of 38 [kN] and a rotation speed of 1800 [rpm].

The total number of revolutions until the inner ring 10 is broken in the axial direction is defined as the inner ring broken life. However, the total number of rotations is 10
^{8 At} [rev], the inner ring splitting life was judged to be sufficient,
The test was stopped.

As can be seen from FIG. 9, when the absolute value of the residual compressive stress is 160 [MPa] or more, a high life of the inner ring splitting life of 10 ⁸ [rev] or more is obtained. It can be used with a margin even under conditions where the fitting stress exceeds 130 [MPa].

From FIGS. 8 and 9, when the contraction rate of the inner ring is 0.25 [%] or more, the absolute value of the residual compressive stress is 160 [MPa] or more, and the inner ring splitting life is 10 [MPa].
^A long life can be obtained when it is ⁸ [rev] or more.

That is, by performing the compressive stress applying step based on the present invention so that the shrinkage ratio of the inner ring becomes 0.25 [%] or more, use under a higher fitting stress exceeding 130 [MPa] is performed. It is possible to provide a rolling bearing having an inner ring capable of withstanding the above.

In the above embodiment, the compressive stress applying step is performed after the quenching. However, a conventional tempering treatment may be separately performed before and after the compressive stress applying step as long as the surface hardness is satisfied.

[0082]

As described above, it is possible to obtain a bearing race ring which is resistant to the generation and propagation of pre-fatigue cracks and thus has an improved fatigue life without post-treatment such as machining after heat treatment. There is an effect that can be.

[Brief description of the drawings]

FIG. 1 is a process chart according to an embodiment of the present invention.

FIG. 2 is a schematic view of an apparatus used in a compressive stress applying step according to the embodiment of the present invention.

FIG. 3 is a diagram showing a relationship between a shrinkage ratio and a residual stress in the first embodiment.

FIG. 4 is a diagram showing a relationship between a shrinkage ratio and a residual stress in a second embodiment.

FIG. 5 is a diagram showing a relationship between a shrinkage ratio and a residual stress in a third embodiment.

FIG. 6 is a diagram showing a state where a bearing is incorporated into an actual alternator.

FIG. 7 is a diagram showing the relationship between bending stress and bearing life in the third embodiment.

FIG. 8 is a diagram illustrating a relationship between a shrinkage ratio and a residual stress in a fourth embodiment.

FIG. 9 is a diagram showing the relationship between inner ring split life and residual stress in a fourth embodiment.

FIG. 10 is a view showing a test cylindrical roller bearing according to a fourth embodiment.

FIG. 11 is a diagram showing a position where a notch 12 and a pre-crack are formed.

FIG. 12 is a diagram for explaining the introduction of a pre-fatigue pre-crack by a three-point bending test machine.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Track ring 2 type 3 Induction heating coil 10 Inner ring 11 Notch 12 Fatigue pre-crack

Claims (1)

[Claims] 1. A method of applying a residual compressive stress to a bearing ring of a quenched bearing by performing tempering while constraining the bearing ring with a mold having a smaller coefficient of thermal expansion than that of the bearing ring to apply a residual compressive stress to the bearing ring. Wherein the heat treatment conditions for the tempering are adjusted to reduce the shrinkage of the bearing ring to 0.1% or more at a shrinkage ratio S defined by the following equation (1). Method for imparting residual compressive stress.